Positive displacement pumps, such as piston pumps and diaphragm pumps, typically displace an essentially constant volume of liquid with each stroke of the positive displacement pumping member. Pumps of this type are very useful for many applications, and a typical positive displacement pump is shown by way of example in Hartley U.S. Pat. No. 4,153,391.
For some applications and under some operating conditions, the characteristic of positive displacement pumps which causes them to displace a constant volume of liquid per stroke is undesirable. For example, when a positive displacement pump is used to supply a liquid for which there is a low demand relative to the output of the pump, the pump must operate under substantial back pressure unless some means is provided to correct this condition. A high back pressure tends to provide a heavy load on the motor for the pump.
One way to attempt to correct this is to cycle the pump on and off in response to demand as is commonly done in water supply systems for recreational vehicles. Unfortunately, the pump and motor may be cycled many times in order to meet the requirements of low liquid demand. A second approach is to bypass the excess liquid, but this requires that the motor and pump operate at maximum speed while producing only a very low effective output. Finally, in certain wobble plate pumps, the angle of the wobble plate can be manually adjusted with appropriate tools to vary the stroke and, hence, the output of the pump. However, manual changes of this type require a relatively long time to make and, therefore, they cannot be made in response to rapidly fluctuating liquid demand.